Device for forming small diameter tubing



June 4, 19.68

s. F. ERICKSON DEVICE FOR FORMING SMALL DIAMETER TUBING Filed June 5, 1964 INVENTOR. 600! E Erickson United States Patent Oflice 3,386,276 Patented June 4, 1968 3,386,276 DEVICE FOR FORMING SMALL DIAMETER TUBING George F. Erickson, Los Alamos, N. Mex., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed June 3, 1964, Ser. No. 372,423 1 Claim. (Cl. 72-56) This invention relates to a method and device for forming small diameter tubing, and in particular, to a method and device that uses a magnetic field produced in a coaxial conductor to form the metal tubing into any desired shape.

This invention makes use of opposing magnetic fields in a coaxial cable to crush the metal tubing against a mandrel which has the preselected desired geometry. The advantage of this process lies in the fact that extremely high pressures are obtainable over long lengths of metal tubing and the nature of the coaxial configuration is such that the magnetic field is uniform over these long lengths. Using this technique and device the inventor has found a method to form small and medium diameter tubing which heretofore was nearly impossible to form. In this process, extremely intricate shapes of small diameter tubing may be produced, these shapes being dictated by the geometry of the mandrel. A further advantage is the freedom from needing a true, permanent, or expandable coil in the usual sense.

In addition to all the aforementioned advantages, the magnetic forming technique of this invention has an additional advantage in that one conductor of the coaxial cable can be of low electrical conductivity and still be formed by the use of a thin copper jacket that is inserted around it to act as a pusher. This method and device also includes a power source which is supplied from a bank of capacitors with any well known switching system.

It is therefore an object of this invention to provide a process for the forming of small diameter metal tubing into any desired shape.

It is a further object of this invention to provide a device for forming small diameter metal tubing into any desired shape.

Embodiment of this invention will now be described by way of example with reference to the accompanying drawing wherein:

The figure is a sectional view of the coaxial cable device in a configuration used for forming the tubing into a smaller diameter tube.

The figure depicts the outer conductor 3 and an inner conductor 1 made of copper which acts as the pusher if a low conducting tube (not shown) is to be formed. Electrical connections are made to the inner copper tube 1 by means of tapered copper plugs 7 and 9 which are driven into the inner tube 1 to produce a good electrical contact. The top plug 7 shorts across the annular gap 2 between the inner and outer conductors thereby completing the electrical current path to ground. The annular gap 2 between the inner 1 and outer 3 conductors must be insulated with dielectric 2 and 10 of sufliciently high breakdown voltage so as to prevent any arcing which might occur between the said conductors. Any arcing which does occur between these conductors significantly decreases the efficiency of the process. Polyvinyl tubing has been found to be an effective means of insulating these conductors; however, this insulation must be held to a minimum thickness if the elficiency of the device is to be maximized. Any dielectric insulating material may be used whose insulating property is such that it will prevent arcing between the inner and outer conductors. The mandrel is held in place in the center of the tube with Micarta plugs 4 which are bored to hold the mandrel. Tight fitting rubber plugs 6 are inserted over the mandrel holders 4 to serve the dual purpose of creating leaktight seals and acting as electrical insulation between the shorting plugs 7 and 9 and the mandrel 5. The coaxial coil is then firmly attached to a coaxial header 11 by means of collets 14 which grip the center conductor through the bottom plate and the outer conductor 3 through the top plate, said plates being electrically insulated from each other by a phenolic spacer 13. This header is fabricated from aluminum and is supplied power by 16 coaxial cables 12 (only 2 of which are shown in this drawing) from a suitable capacitor bank and switching system (not shown). Upon discharging the capacitor bank, the current is carried up the outer conductor 3 and back to ground 15 through the inner conductors 1 and 9. The magnetic fields generated by this high current are repelling to each other causing the inner conductor to be crushed by the magnetic field against the mandrel 5 and the outer conductor 3 to be bulged into a form mold or die cavity (not shown for drawing simplicity). Both the inner and outer conductor can be free formed without restraint if so desired.

The power supply consisting of capacitor bank and suitable switching system should be of a design so that it can supply 10 to 10 amperes for a period of about 10- seconds. Using a power supply capable of supplying such amperage, it is possible to develop instantaneous peak pressures which are in excess of 50,000 pounds per square inch along the length of the tubing.

Operating principles The above described device uses the ability of a copper conductor carrying very high pulse currents (in the range of 10 to 10 amperes) to crush itself due to the magnetic field generated around the conductor. This magnetic field is confined in the annular gap between the inner and outer conductors such that the field acts as if it were a compressed gas tending to crush the inner conductor or to expand the outer conductor depending on the geometry of the coaxial device. The metals which carry the high pulse current should have high electrical conductivities such as silver, copper, or aluminum. If the parts to be formed are of lower conductivity metals, they should be clad, jacketed, or sleeved with high conductivity metals of a thickness approximately equal to one skin depth at the ringing frequency, In this application of the invention, the high conductivity cladding is positioned internally and adjacent to the outer low conducting tube and externally and adjacent to the inner tube. The ringing frequency is determined by the inductance of the coaxial pinch device along with the stray inductance of the cables and the capacity of the energy storage capacitors. The spacing between the inner and outer conductors should be as small a possible to minimize inductance, and when bulge forming the outer conductors, the forces acting on the outer conductor decrease as the spacing in the annular gap increases. Electrical insulators can be clad with metals or compressed in the same manner as low conductivity metals by using high conductivity metals as the pusher on the current carrying member. The time scale for the forming operation is of the order of 10- to 10 seconds, determined by the ringing frequency of the equipment. The energy stored in the capacitor bank is of the order of 10 to 10 joules or watt-seconds.

After the small diameter tubing has been formed using the above described device and technique, the mandrel, if any, which can be composed of a fusible metal alloy, a chemically dissimilar material, plastic, or any material with suitable structural strength, is removed by any suitabel dissolving or melting process. Examples of mandrel materials used in this invention include steel, aluminum,

a brass, copper, phenolic, hard woods, alumina, and berylliar The foregoing illustrations of the present invention are not intended to limit its scope which is to be limited entirely by the appended claim.

What is claimed is:

1. A device for forming small diameter metal tubing comprising in combination:

(a) an inner electrical coaxial conductor,

(b) an outer electrical coaxial conductor which is electrically insulated from said inner conductor by a thin suitable dielectric material,

(0) a power supply capable of supplying 10 to 10 amperes,

the outer conductor to the inner conductor so as to References Cited UNITED STATES PATENTS Harvey et al 7256 Birdsall et al -n 7256 Stinger 7256 Ducati 219-l49 Furth 72-56 o RICHARD J. HERBST, Primary Examiner. 

1. A DEVICE FOR FORMING SMALL DIAMETER METAL TUBING COMPRISING IN COMBINATION: (A) AN INNER ELECTRICAL COAXIAL CONDUCTOR, (B) AN OUTER ELECTRICAL COAXIAL CONDUCTOR WHICH IS ELECTRICALLY INSULATED FROM SAID INNER CONDUCTOR BY A THIN SUITABLE DIELECTRIC MATERIAL, (C) A POWER SUPPLY CAPABLE OF SUPPLYING 10**4 TO 10**6 AMPERES, (D) TAPERED SHORTING PLUGS WHICH ELECTRICALLY CONNECT THE OUTER CONDUCTOR TO THE INNER CONDUCTOR SO AS TO ALLOW CURRENT TO PASS FROM THE POWER SUPPLY TO GROUND BY THE TRIGGERING OF A SUITABLE SWITCHING MEANS, AND (E) A MANDREL THAT IS CONCENTRICALLY POSITIONED WITHIN THE INNER CONDUCTOR AND ELECTRICALLY INSULATED FROM IT. 